Precast concrete structural members are widely used throughout the building industry because of the structural properties, ready availability and low costs of the members. The precast members are typically used to construct decks, such as roofs or floors, in large concrete structural members, such as parking garages. The precast members are manufactured in a facility and then shipped to the job site and erected. The precast members are typically double tee concrete structures, as illustrated in FIG. 6.
Each double tee member has a slab or load bearing surface and includes two flanged edges and two joists. To form a deck, long span, double tee precast members are laid side-by-side one another so that the flanged edges of members are abutting. These members may move relative to one another due to wind forces, thermal expansion and load changes. To prevent or lessen the relative horizontal and vertical movement between the abutting members and to provide added strength and rigidity to the structure, metal pieces may be embedded into the flanged edges of the members. Opposing metal pieces are then welded together with a lug or rod to provide the joinder. The metal pieces are commonly called weldments, weld plates or flange connectors.
At present, typical flange connectors are formed of one-piece metal members comprising (1) a front central plate having a planar weldable surface along one edge of the central plate member and (2) a pair of outstanding arms extending from the central plate member that are embedded in the concrete slab. The flange connectors are cast into the flanged edges of the double tee concrete structure typically at four to five foot centers, varying upon the size of the double tee structure and the amount of expected loading of the structure. The flanged edges are cast in the concrete structure such that the top edge of the central plate member of the flange connector is exposed. Exposing the top edge is accomplished by blocking out a portion of the flanged edge of the concrete member just above the central plate member. Having the top edge exposed allows two adjacent connectors to be welded to a lug or rod positioned between the two adjacent connectors, thereby developing a diaphragm across the floor or roof to increase the rigidity of such floor or roof. Because the opposing connectors often do not align perfectly with one another, a lug or rod is positioned between the opposing connectors. Rather than being welded directly to one another, the connectors are then welded to the intermediary lug or rod.
Examples of such typical flange connectors can be found in Ehlenbeck, U.S. Pat. No. 3,958,954, Lowndes, III, U.S. Pat. No. 4,724,649 and Klein, U.S. Pat. No. 5,402,616. The main problem with the prior art flange connectors, such as those taught by these patents, is that the flange connectors do not accommodate tension, are very rigid and fail dynamically, meaning that there is little deformation in the steel prior to failure and therefore, the failure is difficult if not impossible to anticipate.
Additionally, because concrete cannot handle tension, the precast concrete members are formed with reinforced mesh in the flanged edges of the members. The reinforced mesh is embedded into the center of the concrete slabs and is typically positioned to extend substantially parallel to the planar surface of the slabs.
To best absorb and transmit the forces on the flange connector, the reinforced mesh should be positioned not only in the middle of the concrete slab, but also in alignment with the center of the central plate of the flange connector. Thus, the location of the flange connector relative to the reinforced mesh is quite critical. Typically, the pair of outstanding arms of a flange connector are secured to the mesh or used to support the mesh and also act to align the flange connector with the mesh.
For example, in the case of Klien, U.S. Pat. No. 5,402,616, the outstanding arms are both positioned underneath the reinforced mesh to support the reinforced mesh and hold it in position while the concrete is being cast. The problem with using both outstanding arms to support the underside of the reinforced mesh is that the flange connector is only able to adequately absorb the shear force exerted in the downward direction. Without any means for absorbing the force in the upward direction, the quick flexure that occurs from the rapid changes in loading on a deck, especially in a parking lot, causes failure on the underside of the concrete structure.
Additionally, none of the prior art flange connectors that have outstanding arms provide for the expansion of the central faceplate during welding. The arms typically extend directly outward from the central plate at an approximately forty-five degree (45.degree.) angle. By having the arms extending directly outward from the central plate at forty-five degree (45.degree.) angles, the angles function to compress the faceplate, thereby making it difficult for the faceplate to expand without cracking the surrounding concrete during welding. Finally, none of the prior art flange connectors have been suitable for withstanding seismic loading conditions and dynamic forces without dynamic failure.